Jeffrey Manber, chief executive of NanoRacks, discussed the results of the study, part of NASA’s Next Space Technologies for Exploration Partnerships 2 (NextSTEP-2) effort, in panel discussions Dec. 6 at the SpaceCom Expo here.

The concept, called Ixion, involves taking an Atlas 5 upper stage left in low Earth orbit after a launch and refitting it with life support and other systems needed to support a crew. That converted module could then be attached to the ISS or be used as part of a standalone commercial space station.

The idea of converting an upper stage into a space station module is not a new one. Manber noted that, half a century ago, NASA studied the so-called “wet workshop” approach as one approach for what would become the Skylab space station, but concluded it was too difficult.

Technological improvements have changed that assessment. “We’re now increasingly confident that we can show NASA that this second stage can be repurposed both using crew or even, amazing to me, doing it robotically,” Manber said.

That's not the only possible configuration, at least according to science fiction writers. I seem to recall the Tether Space Station from David Brin's 1983 story Tank Farm Dynamo:

The tank was the next to last in a row of forty of the great cylinders, nestled side by side. A parallel deck of sixteen huge tanks lay about sixty kilometers "overhead" linked to this collection by six strong cables. Twenty meters away from where I stood, one of the half-inch polymer tethers rose from its anchor point, a mirror-bright streak toward the planet overhead.